With certain windshield geometrics, it is not possible for a wiper blade that maintains a fixed angular position relative to the wiper arm to closely match the windshield edges at both the outer and inner limits of the normal wipe pattern. If the wiper is arranged so that the blade matches the side edge of the windshield at outwipe, it will not be parallel to the lower edge of the windshield at inwipe. With a fixed blade-arm geometry, the closest that the wiper blade can come to the lower edge at inwipe will be the point where the inner end of the wiper blade support begins to hit. If it is desired to park the wiper lower than that when the wiper system is turned off, then the wiper blade support will have to be able to change its angular position relative to the wiper arm as the wiper arm rotates farther down.
Drag link mechanisms are known that will change the angular position of the wiper blade support relative to the arm, either continuously, or at the wipe pattern limits. Such mechanisms are often considered unsuitable for passenger cars, because of the very visible extra link, and are typically found only on commercial vehicles. Another known means for allowing the wiper blade to change its angular position as the wiper parks is a coil or tension spring that allows the wiper blade to pivot on the wiper arm when the blade is forced into the lower edge of the windshield molding at park. This is more compact and less visually objectionable than a drag link mechanism. However, such a spring mechanism must be made quite strong in order to not yield appreciably just from the blade friction experienced during normal wiping. Consequently, a good deal of torque must be applied at park to compress the spring, and the wiper mechanism is subjected to that force continually when the wiper is parked.